1. Field of the Invention
This invention relates to an inspection method and apparatus for joint junction states directed to inspect whether soldering is achieved steadily at soldered portions where electrical parts of a large scale integrated (LSI) circuit are soldered to a circuit board.
2. Description of the Prior Art
Defective soldering in electical products leads directly to defective products and must not be permitted absolutely.
Especially, in a large scale electronic computer in which a great number of parts are incorporated at high density, the number of soldered portions increases drastically and all of the joint portions must be inspected for the sake of improving reliability of a product.
Since producibility must also be improved simultaneously with the improvement in reliability, it is necessary to efficiently inspect a great number of soldered portions.
For these reasons, there is at present increasing need for automatic inspection of soldered portions.
However, difficulties are encountered in inspecting soldered portions especially of flat package type parts.
For example, where as shown in FIG. 1c a first flat object 3 typically representative of a soldered portion 1 of a flat package type part shown in FIG. 1a or a wire bonding portion 2 of a large scale integrated circuit shown in FIG. 1b is jointed by soldering to a second object 4, difficulties are faced in inspecting quality of a soldered joint at a junction 5.
Specifically, defects at the soldered portion include complete separation of the first and second objects at the junction, an incomplete joint of the first and second objects represented by mere contact with each other, and displacement of the first and second objects at the junction.
In the flat junction as typically exemplified in FIG. 1c, the complete separation and the incomplete joint equivalent to mere contact are apparently observed as a perfect joint in external appearance and it is very difficult to determine that a defect is present at the junction.
Conventionally, two methods to be described below have been available for inspection of soldered portions of flat package parts.
According to a first method, a soldered portion is vibrated at a frequency of 60 Hz to 200 KHz by means of a vibrator which is brought into direct contact with the soldered portion, and an excited vibration of the soldered portion is detected by a vibration sensor so that the presence or absence of defect in soldering is judged on the basis of a detected vibratory status (U.S. Pat. No. 4,218,922).
According to a second method, a soldered portion is vibrated at varying frequencies within a range of from 20 Hz to 1 MHz or from 150 KHz to 650 KHz by means of a vibrator which is brought into direct contact with the soldered portion, and the magnitude of an excited vibration of the soldered portion is detected by a vibration sensor to measure a frequency response of the soldered portion so that the presence or absence of defect in the soldered portion is judged on the basis of the measured frequency response (U.S. Pat. No. 4,287,766).
These methods face however the following problems. More particularly, since in the aforementioned prior art methods the vibrator is brought into direct contact with the soldered portion to vibrate the soldered portion and the excited vibration is detected by the vibration sensor,
(1) The vibrator must be accurately brought into contact with every soldered portion and as a result, the inspection speed slows down; and
(2) It is difficult to keep constant the contact state between the soldered portion and the vibrator as well as the vibration sensor, and hence reliability tends to be degraded.
Accordingly, the prior art methods connot be adapted for the large scale electronic computer which incorporates a great number of parts at high density and therefore, urgent development of inspection method and apparatus which meet the requisite reliability and producibility has been desired.